Transversely-excited film bulk acoustic resonator using yx-cut lithium niobate for high power applications

ABSTRACT

Acoustic resonator devices, filters, and methods are disclosed. An acoustic resonator includes a substrate and a lithium niobate (LN) plate having front and back surfaces and a thickness ts. The back surface is attached to a surface of the substrate. A portion of the LN plate forms a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the LN plate with interleaved fingers of the IDT disposed on the diaphragm. The LN plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic wave in the diaphragm. Euler angles of the LN plate are [0°, β, 0° ], where 0≤β≤60°. A thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 ts and less than or equal to 2.0 ts.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application 63/026,824, filed May 19, 2020, entitled IDT SIDEWALL ANGLE TO CONTROL SPURIOUS MODES IN XBARS.

This patent is a continuation-in-part of application Ser. No. 16/829,617, entitled HIGH POWER TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS ON Z-CUT LITHIUM NIOBATE, filed Mar. 25, 2020, which is a continuation of application Ser. No. 16/578,811, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS FOR HIGH POWER APPLICATIONS, filed Sep. 23, 2019, now U.S. Pat. No. 10,637,438. This patent is also a continuation in part of application Ser. No. 16/782,971, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR USING ROTATED Y-X CUT LITHIUM NIOBATE, filed Feb. 5, 2020, which claims priority from 62/904,133, filed Sep. 23, 2019, entitled WIDE BAND BAW RESONATORS ON 120-130 Y-X LITHIUM NIOBATE SUBSTRATES, is a continuation-in-part of application Ser. No. 16/689,707, entitled BANDPASS FILTER WITH FREQUENCY SEPARATION BETWEEN SHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYER THICKNESS, filed Nov. 20, 2019, and is a continuation in part of application Ser. No. 16/438,141, filed Jun. 11, 2019, entitled SOLIDLY-MOUNTED TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No. 10,601,392, which claims priority from application 62/818,564, filed Mar. 14, 2019, entitled SOLIDLY MOUNTED XBAR and application 62/753,809, filed Oct. 31, 2018, entitled SOLIDLY MOUNTED SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. Application Ser. No. 16/438,141 is also a continuation-in-part of application Ser. No. 16/230,443. Application Ser. No. 16/578,811 is a continuation-in-part of application Ser. No. 16/230,443 and application Ser. No. 16/689,707 is a continuation of application Ser. No. 16/230,443. Application Ser. No. 16/230,443, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192, claims priority from the following provisional patent applications: application 62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. All of these applications are incorporated herein by reference.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. The current LTE™ (Long Term Evolution) specification defines frequency bands from 3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposals for wireless communications include millimeter wave communication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic plan view and two schematic cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of the XBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR of FIG. 1.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in an XBAR.

FIG. 5 is a schematic block diagram of a filter using XBARs.

FIG. 6 is a schematic cross-sectional view of two XBARs illustrating a frequency-setting dielectric layer.

FIG. 7 is a chart of the e14 and e15 piezoelectric coefficients of a lithium niobate plate with Euler angles [0°, β, 0° ] as functions of γ.

FIG. 8 is a chart comparing the admittances of XBARs formed on rotated Y-X lithium niobate and Z cut lithium niobate.

FIG. 9 is a graph identifying preferred combinations of aluminum IDT thickness and IDT pitch for XBARs using rotated Y-X cut lithium niobate without a front-side dielectric layer.

FIG. 10 is a graph identifying preferred combinations of aluminum IDT thickness and IDT pitch for XBARs using rotated Y-X cut lithium niobate with a front side dielectric layer with a thickness of 0.2 times a thickness of the piezoelectric diaphragm.

FIG. 11 is a graph identifying preferred combinations of aluminum IDT thickness and IDT pitch for XBARs using rotated Y-X cut lithium niobate with a front side dielectric layer with a thickness of 0.3 times a thickness of the piezoelectric diaphragm.

FIG. 12 is a graph identifying preferred combinations of aluminum IDT thickness and IDT pitch for XBARs using rotated Y-X cut lithium niobate with a front side dielectric layer with a thickness of 0.35 times a thickness of the piezoelectric diaphragm.

FIG. 13 is a flow chart of a process for fabricating an acoustic resonator or filter using rotated Y-X cut lithium niobate.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonal cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR) 100. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having a front surface 112 and a back surface 114. The front and back surfaces are essentially parallel. “Essentially parallel” means parallel to the extent possible within normal manufacturing tolerances. The piezoelectric plate 110 is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate 110 is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples presented in this patent, the piezoelectric plates 110 are Z-cut, which is to say the Z axis is normal to the front surface 112 and back surface 114. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations including rotated Z-cut and rotated YX-cut.

The back surface 114 of the piezoelectric plate 110 is attached to a surface 122 of the substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate 120. The portion of the piezoelectric plate 110 that spans the cavity is referred to herein as the “diaphragm” due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be attached to the surface 122 of the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or otherwise attached to the substrate. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers.

The cavity 140 is an empty space within a solid body of the XBAR 100. The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120 (as shown subsequently in FIG. 3). The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. An IDT is an electrode structure for converting between electrical and acoustic energy in piezoelectric devices. The IDT 130 includes a first plurality of parallel elongated conductors, commonly called “fingers”, such as finger 136, extending from a first busbar 132. The IDT 130 includes a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the first and second sets of fingers in an IDT. As shown in FIG. 1, each busbar 132, 134 is an elongated rectangular conductor with a long axis orthogonal to the interleaved fingers and having a length approximately equal to the length L of the IDT. The busbars of an IDT need not be rectangular or orthogonal to the interleaved fingers and may have lengths longer than the length of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. As will be discussed in further detail, the primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the diaphragm 115 of the piezoelectric plate that spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may more or fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device will have more than ten parallel fingers in the IDT 110. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated in the drawings.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100. The piezoelectric plate 110 is a single-crystal layer of piezoelectrical material having a thickness ts. ts may be, for example, 100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g. bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000 nm.

A front-side dielectric layer 214 may be formed on the front side of the piezoelectric plate 110. The “front side” of the XBAR is the surface facing away from the substrate. The front-side dielectric layer 214 has a thickness tfd. The front-side dielectric layer 214 is formed between the IDT fingers 238. Although not shown in FIG. 2, the front side dielectric layer 214 may also be deposited over the IDT fingers 238. A back-side dielectric layer 216 may be formed on the back side of the piezoelectric plate 110. The back-side dielectric layer 216 has a thickness tbd. The front-side and back-side dielectric layers 214, 216 may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal, and the front-side and back-side dielectric layers 214, 216 are not necessarily the same material. Either or both of the front-side and back-side dielectric layers 214, 216 may be formed of multiple layers of two or more materials.

The IDT fingers 238 may be one or more layers of aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, beryllium, gold, molybdenum, or some other conductive material. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT may be made of the same or different materials as the fingers. As shown in FIG. 2, the IDT fingers 238 have rectangular cross-sections. The IDT fingers may have some other cross-sectional shape, such as trapezoidal.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or “mark” of the IDT fingers. The IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric slab 212. The width of the IDT fingers in an XBAR is not constrained to one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to about equal to the width w. The thickness of the busbars (132, 134 in FIG. 1) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers.

FIG. 3 is an alternative cross-sectional view along the section plane A-A defined in FIG. 1. In FIG. 3, a piezoelectric plate 310 is attached to a substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate. The cavity 340 does not fully penetrate the substrate 320. Fingers of an IDT are disposed on the diaphragm 315. The cavity 340 may be formed, for example, by etching the substrate 320 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric plate 310. In this case, the diaphragm 315 may be contiguous with the rest of the piezoelectric plate 310 around a large portion of a perimeter 345 of the cavity 340. For example, the diaphragm 315 may be contiguous with the rest of the piezoelectric plate 310 around at least 50% of the perimeter 345 of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode of interest in an XBAR. FIG. 4 shows a small portion of an XBAR 400 including a piezoelectric plate 410 and three interleaved IDT fingers 430. A radio frequency (RF) voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is primarily lateral, or parallel to the surface of the piezoelectric plate 410, as indicated by the arrows labeled “electric field”. Since the dielectric constant of the piezoelectric plate is significantly higher than the surrounding air, the electric field is highly concentrated in the plate relative to the air. The lateral electric field introduces shear deformation, and thus strongly excites a shear-mode acoustic mode, in the piezoelectric plate 410. Shear deformation is deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. A “shear acoustic mode” is an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR 400 are represented by the curves 460, with the adjacent small arrows providing a schematic indication of the direction and magnitude of atomic motion. The degree of atomic motion, as well as the thickness of the piezoelectric plate 410, have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown in FIG. 4), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances can achieve better performance than current state-of-the art film-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices where the electric field is applied in the thickness direction. In such devices, the acoustic mode is compressive with atomic motions and the direction of acoustic energy flow in the thickness direction. In addition, the piezoelectric coupling for shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. High piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with appreciable bandwidth.

FIG. 5 is a schematic circuit diagram and layout for a high frequency band-pass filter 500 using XBARs. The filter 500 has a conventional ladder filter architecture including three series resonators 510A, 510B, 510C and two shunt resonators 520A, 520B. The three series resonators 510A, 510B, and 510C are connected in series between a first port and a second port (hence the term “series resonator”). In FIG. 5, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 500 is bidirectional and either port may serve as the input or output of the filter. The two shunt resonators 520A, 520B are connected from nodes between the series resonators to ground. A filter may contain additional reactive components, such as inductors, not shown in FIG. 5. All the shunt resonators and series resonators are XBARs. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all of the series resonators are connected in series between an input and an output of the filter. All of the shunt resonators are typically connected between ground and the input, the output, or a node between two series resonators.

In the exemplary filter 500, the three series resonators 510A, B, C and the two shunt resonators 520A, B of the filter 500 are formed on a single plate 530 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In FIG. 5, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 535). In this example, each IDT is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a single cavity.

Each of the resonators 510A, 510B, 510C, 520A, 520B in the filter 500 has resonance where the admittance of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter 500. In over-simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti-resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In a typical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter's passband and the anti-resonance frequencies of the series resonators are position above the upper edge of the passband.

FIG. 6 is a schematic cross-sectional view through a shunt resonator and a series resonator of a filter 600 that uses a dielectric frequency setting layer to separate the resonance frequencies of shunt and series resonators. A piezoelectric plate 610 is attached to a substrate 620. Portions of the piezoelectric plate 610 form diaphragms spanning cavities 640 in the substrate 620. Interleaved IDT fingers, such as finger 630, are formed on the diaphragms. A first dielectric layer 650, having a thickness t1, is formed over the IDT of the shunt resonator. The first dielectric layer 650 is considered a “frequency setting layer”, which is a layer of dielectric material applied to a first subset of the resonators in a filter to offset the resonance frequencies of the first subset of resonators with respect to the resonance frequencies of resonators that do not receive the dielectric frequency setting layer. The dielectric frequency setting layer is commonly SiO₂ but may be silicon nitride, aluminum oxide, or some other dielectric material. The dielectric frequency setting layer may be a laminate or composite of two or more dielectric materials.

A second dielectric layer 655, having a thickness t2, may be deposited over both the shunt and series resonator. The second dielectric layer 655 serves to seal and passivate the surface of the filter 600. The second dielectric layer may be the same material as the first dielectric layer or a different material. The second dielectric layer may be a laminate or composite of two or more different dielectric materials. Further, as will be described subsequently, the thickness of the second dielectric layer may be locally adjusted to fine-tune the frequency of the filter 600. Thus, the second dielectric layer can be referred to as the “passivation and tuning layer”.

The resonance frequency of an XBAR is roughly proportional to the inverse of the total thickness of the diaphragm including the piezoelectric plate 610 and the dielectric layers 650, 655. The diaphragm of the shunt resonator is thicker than the diaphragm of the series resonator by the thickness t1 of the dielectric frequency setting layer 650. Thus, the series resonator will have a lower resonance frequency than the shunt resonator. The difference in resonance frequency between series and shunt resonators is determined by the thickness t1.

This patent is directed to XBAR devices on lithium niobate plates having Euler angles [0°, β, 0° ]. For historical reasons, this plate configuration is commonly referred to as “Y-cut”, where the “cut angle” is the angle between the y axis and the normal to the plate. The “cut angle” is equal to β+90°. For example, a plate with Euler angles [0°, 30°, 0° ] is commonly referred to as “120° rotated Y-cut”.

FIG. 7 is a graph 700 of two piezoelectric stress coefficients e15 and e16 for lithium niobate plates having Euler angles [0°, β, 0° ]. The solid line 710 is a plot of piezoelectric stress coefficient e15 relating electric field along the x axis to shear stress or torque about the y axis as a function of β. This shear stress excites the shear primary acoustic mode shown in FIG. 4. The dashed line 720 is a plot of piezoelectric stress coefficient e16 relating electric field along the x axis to shear stress or torque about the z axis as a function of β. This shear stress excites horizontal shear modes (e.g. the SHO plate mode) with atomic displacements normal to the plane of FIG. 4, which are undesired parasitic modes in an XBAR. Note that these two curves are identical and shifted by 90°.

Inspection of FIG. 7 shows that the first piezoelectric stress coefficient is highest for Euler angle θ about 30°. The first piezoelectric stress coefficient is higher than about 3.8 (the highest piezoelectric stress coefficient for an unrotated Z-cut lithium niobate) for 0°≤β≤60°. The second piezoelectric stress coefficient is zero for Euler angle θ about 30°, where the first piezoelectric stress coefficient is maximum. In this context “about 30°” means “within a reasonable manufacturing tolerance of 30°”. The second piezoelectric stress coefficient is less than about 10% of the first piezoelectric stress coefficient for 26°≤β≤34°.

FIG. 8 is a chart 800 showing the normalized magnitude of the admittance (on a logarithmic scale) as a function of frequency for two XBAR devices simulated using finite element method (FEM) simulation techniques. The dashed line 820 is a plot of the admittance of an XBAR on a Z-cut lithium niobate plate. In this case, the Z crystalline axis is orthogonal to the surfaces of the plate, the electric field is applied along the Y crystalline axis, and the Euler angles of the piezoelectric plate are 0, 0, 90°. The solid line 810 is a plot of the admittance of an XBAR on a 120° Y-cut lithium niobate plate. In this case, the electric field is applied along the X crystalline axis, which lies in the plane of the surfaces of the lithium niobate plate. The YZ plane is normal to the surfaces of the plate. The Z crystalline axis is inclined by 30° with respect to orthogonal to the surfaces of the plate, and the Euler angles of the piezoelectric plate are 0°, 30°, 0°. In both cases, the plate thickness is 400 nm, and the IDT fingers are aluminum 100 nm thick. The substrate supporting the piezoelectric plate is silicon with a cavity formed under the IDT fingers.

The difference between anti-resonance and resonance frequencies of the resonator on the rotated Y-cut plate (solid line 810) is about 200 MHz greater than the difference between anti-resonance and resonance frequencies of the resonator on the Z-cut plate (dashed line 820). The electromechanical coupling of the XBAR on the rotated Y-cut plate is about 0.32; the electromechanical coupling of the XBAR on the Z-cut plate is about 0.24.

U.S. Pat. No. 10,637,438 describes XBAR resonators for use in high power applications. U.S. Pat. No. 10,637,438 also describes the use of a figure of merit (FOM) to define a design space (i.e. combinations of IDT conductor thickness, pitch, and width) that provides XBARs with acceptable performance for use in filters. The FOM is calculated by integrating the negative impact of spurious modes across a defined frequency range. For each combination of IDT conductor thickness and pitch, the FOM is calculated for a range of IDT finger widths. The minimum FOM value over the range of IDT finger widths is considered the minimized FOM for that conductor thickness/pitch combination. The definition of the FOM and the frequency range depend on the requirements of a particular filter. The frequency range typically includes the passband of the filter and may include one or more stop bands. Spurious modes occurring between the resonance and anti-resonance frequencies of each hypothetical resonator may be accorded a heavier weight in the FOM than spurious modes at frequencies below resonance or above anti-resonance. Hypothetical resonators having a minimized FOM below a threshold value were considered potentially “useable”, which is to say probably having sufficiently low spurious modes for use in a filter. Hypothetical resonators having a minimized cost function above the threshold value were considered not useable.

FIG. 9 is a chart 900 showing combinations of IDT pitch p and IDT finger thickness tm that may provide useable resonators. Both IDT pitch and IDT finger thickness are normalized to the thickness ts of the piezoelectric plate. This chart is based on two-dimensional simulations of XBARs with lithium niobate diaphragms, aluminum conductors, and no dielectric layers. XBARs with IDT pitch and thickness within unshaded regions 910, 920, 930 are likely to have sufficiently low spurious effects for use in filters. XBARs with IDT pitch and thickness within unshaded regions 940, 950, 960 are likely to have sufficiently low spurious effects for use in filters, but the IDT metal thickness is too low for use in high power applications. XBARs with IDT pitch and thickness within the intervening shaded regions have unacceptably high spurious modes for use in the target filter. With no dielectric layers, usable resonators exist for IDT finger thickness greater than or equal to 0.8 times the piezoelectric plate thickness and less than or equal to 2.0 times the piezoelectric plate thickness.

FIG. 10 is a chart 1000 showing combinations of IDT pitch and IDT finger thickness that may provide useable resonators with a front-side dielectric layer having a thickness tfd equal to 0.2 times the piezoelectric plate thickness ts. The front-side dielectric layer may be a frequency setting dielectric layer deposited between the IDT fingers of a subset of resonators in a filter circuit, such as the shunt resonators 520A, 520B in the filter circuit of FIG. 5. In the chart 1000, both IDT pitch and IDT finger thickness are normalized to the thickness of the piezoelectric plate. This chart is based on two-dimensional simulations of XBARs with lithium niobate diaphragms, aluminum conductors, and an SiO₂ front-side dielectric layer. XBARs with IDT pitch and thickness within unshaded regions 1010, 1020, 1030 are likely to have sufficiently low spurious effects for use in filters. XBARs with IDT pitch and thickness within unshaded regions 1040 and 1050 are likely to have sufficiently low spurious effects for use in filters, but the IDT metal thickness is too low for use in high power applications. XBARs with IDT pitch and thickness within the intervening shaded regions have unacceptably high spurious modes for use in the target filter. With a front-side dielectric layer having a thickness equal to 0.2 times the piezoelectric plate thickness, usable resonators exist for IDT finger thickness greater than or equal to 1.1 times the piezoelectric plate thickness and less than or equal to 2.0 times the piezoelectric plate thickness. Usable resonators will exist for this range of IDT finger thickness for front-side dielectric layer thickness less than or equal to 0.2 times the piezoelectric plate thickness.

FIG. 11 is a chart 1100 showing combinations of IDT pitch and IDT finger thickness that may provide useable resonators with a front-side dielectric layer, which may be a frequency setting dielectric layer, having a thickness equal to 0.3 times the piezoelectric plate thickness. Both IDT pitch and IDT finger thickness are normalized to the thickness of the piezoelectric plate. This chart is based on two-dimensional simulations of XBARs with lithium niobate diaphragms, aluminum conductors, and an SiO₂ front-side dielectric layer. XBARs with IDT pitch and thickness within unshaded regions 1110, 1120, 1130 are likely to have sufficiently low spurious effects for use in filters. XBARs with IDT pitch and thickness within the intervening shaded regions have unacceptably high spurious modes for use in the target filter. Usable XBARs with thin IDT conductors do not exist. With a front-side dielectric layer having a thickness equal to 0.3 times the piezoelectric plate thickness, usable resonators exist for IDT finger thickness greater than or equal to 1.15 times the piezoelectric plate thickness and less than or equal to 1.8 times the piezoelectric plate thickness. Usable resonators will exist for this range of IDT finger thickness for front-side dielectric layer thickness greater than 0.2 times the piezoelectric plate thickness and less than or equal to 0.3 times the piezoelectric plate thickness.

FIG. 12 is a chart 1200 showing combinations of IDT pitch and IDT finger thickness that may provide useable resonators with a front-side dielectric layer having a thickness equal to 0.35 times the piezoelectric plate thickness. Both IDT pitch and IDT finger thickness are normalized to the thickness of the piezoelectric plate. This chart is based on two-dimensional simulations of XBARs with lithium niobate diaphragms, aluminum conductors, and an SiO₂ front-side dielectric layer. XBARs with IDT pitch and thickness within a small unshaded region 1210 will have acceptably low spurious modes for use in filters. XBARs with IDT pitch and thickness within the surrounding shaded regions have unacceptably high spurious modes for use in the target filter. Usable XBARs with thin IDT conductors do not exist. 0.35 times the piezoelectric plate thickness is an upper limit for front side dielectric thickness. No useful XBARs exist for materially thicker dielectric layers.

Description of Methods

FIG. 13 is a simplified flow chart showing a process 1300 for making an XBAR or a filter incorporating XBARs. The process 1300 starts at 1305 with a substrate and a plate of piezoelectric material and ends at 1395 with a completed XBAR or filter. The flow chart of FIG. 13 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 13.

The flow chart of FIG. 13 captures three variations of the process 1300 for making an XBAR which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps 1310A, 1310B, or 1310C. Only one of these steps is performed in each of the three variations of the process 1300.

The piezoelectric plate may be, for example, rotated Y-cut lithium niobate. The Euler angles of the piezoelectric plate are [0°, β, 0° ], where β is in the range from 0° to 60°. Preferably, β may be in the range from 26° to 34° to minimize coupling into shear horizontal acoustic modes. β may be about 30°. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the process 1300, one or more cavities are formed in the substrate at 1310A, before the piezoelectric plate is bonded to the substrate at 1320. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 1310A will not penetrate through the substrate.

At 1320, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric plate and the substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or intermediate material layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1330 by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. A conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the conductor pattern (for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1330 by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1330 using a lift-off process. Photoresist may be deposited over the piezoelectric plate and patterned to define the conductor pattern. The conductor layer and, optionally, one or more other layers may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern.

At 1340, a front-side dielectric layer may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. The one or more dielectric layers may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. The one or more dielectric layers may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate.

In a second variation of the process 1300, one or more cavities are formed in the back side of the substrate at 1310B. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown in FIG. 1.

In the second variation of the process 1300, a back-side dielectric layer may be formed at 1350. In the case where the cavities are formed at 1310B as holes through the substrate, the back-side dielectric layer may be deposited through the cavities using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 1300, one or more cavities in the form of recesses in the substrate may be formed at 1310C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in a filter device.

In all variations of the process 1300, the filter device is completed at 1360. Actions that may occur at 1360 include depositing an encapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 1360 is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 1395.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. An acoustic resonator device comprising: a substrate having a surface; a lithium niobate plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the lithium niobate plate forming a diaphragm that spans a cavity in the substrate; and an interdigital transducer (IDT) formed on the front surface of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein the lithium niobate plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm, Euler angles of the lithium niobate plate are [0°, β, 0° ], where β is greater than or equal to 0° and less than or equal to 60°, and a thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 times a thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.
 2. The device of claim 1, wherein β is greater than or equal to 26° and less than or equal to 34°.
 3. The device of claim 1, wherein β is about 30°.
 4. The device of claim 1, further comprising a dielectric layer formed between the interleaved fingers of the IDT.
 5. The device of claim 4, wherein a thickness of the dielectric layer is less than or equal to 0.2 times a thickness of the lithium niobate plate, and a thickness of the interleaved fingers of the IDT is greater than or equal to 1.1 times a thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.
 6. The device of claim 4, wherein a thickness of the dielectric layer is greater than 0.2 times a thickness of the lithium niobate plate and less than or equal to 0.3 times a thickness of the lithium niobate plate, and a thickness of the interleaved fingers of the IDT is greater than or equal to 1.15 times a thickness of the lithium niobate plate and less than or equal to 1.8 times the thickness of the lithium niobate plate.
 7. The device of claim 4, wherein a thickness of the dielectric layer is less than or equal to 0.35 times a thickness of the lithium niobate plate.
 8. The device of claim 1, wherein a direction of acoustic energy flow of the primary acoustic mode is substantially orthogonal to the front and back surfaces of the diaphragm.
 9. A filter device, comprising: a substrate having a surface; a lithium niobate plate having front and back surfaces, the back surface attached to the surface of the substrate, portions of the lithium niobate plate forming one or more diaphragms spanning respective cavities in the substrate; and a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, interleaved fingers of each of the plurality of IDTs disposed on respective diaphragms of the one or more diaphragms, wherein the lithium niobate plate and all of the IDTs are configured such that respective radio frequency signals applied to the IDTs excite respective primary shear acoustic modes in the respective diaphragms, Euler angles of the lithium niobate plate are [0°, β, 0° ], where β is greater than or equal to 0° and less than or equal to 60°, and the interleaved fingers of all of the IDTs have a common thickness greater than or equal to 0.8 times a thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.
 10. The filter device of claim 9, wherein β is greater than or equal to 26° and less than or equal to 34°.
 11. The filter device of claim 9, wherein β is about 30°.
 12. The filter device of claim 9, further comprising a frequency setting dielectric layer formed between the interleaved fingers of a subset of the plurality of IDTs.
 13. The filter device of claim 12, wherein a thickness of the frequency setting dielectric layer is less than or equal to 0.2 times a thickness of the lithium niobate plate, and the common thickness of the interleaved fingers of the IDTs is greater than or equal to 1.1 times a thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.
 14. The filter device of claim 12, wherein a thickness of the frequency setting dielectric layer is greater than 0.2 times a thickness of the lithium niobate plate and less than or equal to 0.3 times a thickness of the lithium niobate plate, and the common thickness of the interleaved fingers of the IDTs is greater than or equal to 1.15 times a thickness of the lithium niobate plate and less than or equal to 1.8 times the thickness of the lithium niobate plate.
 15. The filter device of claim 12, wherein a thickness of the frequency setting dielectric layer is less than or equal to 0.35 times a thickness of the lithium niobate plate.
 16. The filter device of claim 12, wherein the plurality of acoustic resonators includes one or more shunt resonators and one or more series resonator connected in a ladder filter circuit, and the subset of the plurality of IDTs is the one or more shunt resonators.
 17. The filter device of claim 9, wherein respective directions of acoustic energy flow of all of the primary acoustic modes are substantially orthogonal to the front and back surfaces of the diaphragm.
 18. The filter device of claim 9, wherein interleaved fingers of each of the plurality of IDTs are disposed on a respective diaphragm spanning a respective cavity.
 19. A method of fabricating an acoustic resonator device, comprising: bonding a back surface of a lithium niobate plate to a substrate such that a portion of the lithium niobate plate forms a diaphragm spanning a cavity in the substrate; and forming an interdigital transducer (IDT) on a front surface of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein the lithium niobate plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm, Euler angles of the lithium niobate plate are [0°, β, 0° ], where β is greater than or equal to 0° and less than 60°, and a thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 times a thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.
 20. The method of claim 19, wherein β is greater than or equal to 26° and less than or equal to 34°.
 21. The method of claim 19, wherein β is about 30°. 